In recent years, fuel cells have been attracting attention as high-efficiency energy conversion devices. Fuel cells are roughly classified, based on the type of the electrolyte used, into low-temperature operating fuel cells such as alkaline fuel cells, solid polymer electrolyte fuel cells, and phosphoric acid fuel cells, and high-temperature operating fuel cells such as molten carbonate fuel cells and solid oxide fuel cells. Among them, the solid polymer electrolyte fuel cell (PEFC) that uses an ionically conductive polymer electrolyte membrane as an electrolyte has been receiving attention as a power source for stationary use, automotive use, portable use, etc., because it is compact in construction, achieves high output density, does not use a liquid for the electrolyte, can operate at low temperatures, and can therefore be implemented in a simple system.
The basic principle of the solid polymer electrolyte fuel cell is that, with one side of the polymer electrolyte membrane exposed to a fuel gas (hydrogen or the like) and the other side to an oxidizer gas (air or the like), water is synthesized by a chemical reaction occurring across the polymer electrolyte membrane, and the resulting reaction energy is extracted as electrical energy. FIG. 1 is an exploded perspective view showing the structure of a conventional fuel cell, and FIG. 2 is a cross-sectional side view of its assembly. In FIGS. 1 and 2, reactant gases introduced through a gas flow passage formed in a separator pass through the polymer electrolyte membrane and cause electrochemical reactions to occur on porous catalytic electrodes, and the power generated here is recovered outside through the separator. As is apparent from this structure, the polymer electrolyte membrane and the porous catalytic electrodes must be physically joined together. A structure constructed by placing the porous catalytic electrodes on both sides of the polymer electrolyte membrane and forming them into an integral structure by thermal pressing or the like is generally called a membrane electrode assembly (MEA). Each MEA can be handled independently, and a gasket is placed between the MEA and the separator to prevent reactant gases from leaking outside. The polymer electrolyte membrane has ionic conductivity, and has the function of physically and electronically isolating the fuel electrode from the oxygen electrode because of its lack of air permeability and electron conductivity. If the size of the polymer electrolyte membrane is smaller than that of each porous catalytic electrode, electrical short-circuiting occurs between the porous catalytic electrodes within the MEA, and further, the oxidizer gas and the fuel gas mix together (cross leaking), resulting in the loss of its function as a cell. Accordingly, the area size of the polymer electrolyte membrane must be made the same as or larger than that of each porous catalytic electrode. In view of this, usually the polymer electrolyte membrane is formed extending beyond the edges of the porous catalytic electrodes, and a gas sealing and supporting structure is formed by sandwiching it between the gasket and the separator.
Since the polymer electrolyte membrane is formed from an extremely thin film material, the membrane is difficult to handle, and its peripheral edge which is important for reactant gas sealing may often become wrinkled, for example, when joining it to the electrodes or when assembling a plurality of unit cells to fabricate a cell stack. In a unit cell or a cell stack fabricated using such a wrinkled polymer electrolyte membrane, there is a high probability that reactant gases will leak out through the wrinkled portions. Even if the polymer electrolyte membrane is free from such wrinkles, the polymer electrolyte membrane is prone to damage as it is a component element having the least mechanical strength of all the component elements forming the stack. Accordingly, it is desired to reinforce the polymer electrolyte membrane structure in order to enhance the reliability, maintainability, etc. of the solid polymer electrolyte fuel cell. Furthermore, to prevent short-circuiting at the edge of the polymer electrolyte membrane, conventional MEAs have been made that incorporate a polymer electrolyte membrane having a larger area than the electrode layers, with the electrolyte membrane extending laterally beyond the edge of the electrode layers. However, when fabricating an MEA by using such an electrolyte membrane differently sized than the electrode layers, since there is a need to cut them separately and to position them relative to each other, the number of fabrication steps increases, resulting in reduced productivity.
It is known to provide a method for forming a unitized membrane electrode assembly having a thermoplastic polymer, seal against fluid permeation, seal, by applying a thermoplastic polymer by such means as injection molding or compression molding to the edge of an MEA having a polymer electrolyte membrane of the same size as or larger than the gas diffusion electrodes, wherein the thermoplastic polymer is impregnated into the sealing edges of the gas diffusion backings, and the seal envelops a peripheral region of both gas diffusion backings and the polymer electrolyte membrane (Tokuhyou (Published Japanese Translation of PCT Application) No. 2005-516350).
It is also known to provide a method wherein, in order to effectively reinforce the polymer electrolyte membrane and to greatly enhance the handling characteristics of the fuel cell structure, a frame member is press-fitted onto the outer periphery of the porous structures fixed to both sides of the polymer electrolyte membrane, thereby joining the frame member and the porous structures firmly and reliably (Japanese Unexamined Patent Publication No. H10-199551).